Sometimes on a slab home, or a home with a lower level that opens up on the rear, I see the duct work running downward from the HVAC system to service the house.

I point out to my clients that ducts under a slab will very often be an issue.

The system seen here services every room on the lower level of this house. That is the supply duct visible on the left.

Every single register except one blows the air from the floor.

Personally I don't like floor registers even on upper levels because I find them less efficient during air conditioning and they always affect furniture placement. It seems that the registers are put into a room right where you would want the bureau to go! But that is just me.

But aside from that, what's wrong with ducts under a slab?

Many things can result. Among them are:

1. They are inaccessible. If anything goes wrong with them repairs can be difficult and/or expensive.

2. When the heat is operating those ducts are cool. Their temperature varies little. Condensation results. I have seen ducts blowing from under a slab onto a wall which is wet with condensation. People have called me out of the blue, after finding me online, to ask why their walls are wet!

3. The ducts are metal. Metal rusts. This duct is not rusting much yet. But you can see the pitting. Actually it is much better than I expected! Is the rust and pitting worse where the opening is not exposed to drier air?

4. As the duct rusts, it can open up.

I have seen ducts under a slab that have a couple of inches of water in them from the water table outdoors!

I have seen ducts full of mud!

I have seen dead snakes, turtles and frogs inside ducts! No kidding here either.

I have seen ducts that have completely disappeared and what is left is part slab and part soil!

Sometimes as the lower duct material disappears, the soil level can raise as water deposits more material, effectively closing off the duct and creating a half moon of service. THIS IS EXTREMELY INEFFICIENT.

I have seen ducts full of molds, fungus and even mushrooms. As they say, all manner of microbial growth! Can you spell H-E-A-L-T-H-F-U-L boys and girls?

Of course the problem worsens as the house ages.

My recommendation: even on new construction, if you have a property with ducts under the slab, have a peek. See what they are doing. But mention it to your clients. Physics have principles that are always obeyed unless they are overcome. When heat and cold mix, moisture results. Pure and simple. Unless something is in place to control that moisture, water will do what water does.

I see this pretty rarely here(only on a few homes). For the most part they have been dry but that is because our climate and ground moisture in most areas are dryer. But like you I just think it is a bad idea from the aspect if something goes wrong then it is a painful process to repair.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 7 years ago

I have seen these so often, Don, that I am a bit surprised to hear how little they are seen around the country! I do NOT see them in recent construction, however. They learned their lessons I guess. There is not much worthy about this installation.

Have you seen any solutions to fix ducts under the slab. I have a client that is testing a 'sealing' company to see if it helps. He is running into a radon issue that we both agree is coming from the ducts. He has a mitigation system and it has not made a major difference in level of radon in the home. I think the ducts are pushing it in faster than the mitigation system can get it out.

Posted by Charles Hendricks (The Gaines Group, PLC Architecture and Design) over 7 years ago

That's a problem Charles. How can a mitigation system not be taking in air from the ducts as well? All the under-slab ducts I see are pitted, cracked, broken, or otherwise compromised. Most I see are partially closed off. Once they are encroached from below by mud, et al, all that would need to be completely removed before any sealing system could work. This must be a slab on a lower level exposed to the ground.

I have not heard of a sealing system around here. And we are not that far apart! It seems to me that if there is such a system all those having these ducts could really benefit.

What would they seal things with - a rubber membrane? Rubber spray? It would have to be something without VOCs. It would have to contain zinc or copper to prevent molds from developing. Here I am talking about something that I haven't seen, like I know! But, for sure, those things would need to be in the mix.